Heavy duty material handling vehicles have been in existence for a number of years. One type of such unit is a truck mounted crane wherein a turntable is supported for rotation about a vertical axis on a vehicle chassis frame and the turntable supports a material handling unit, such as a telescopic crane.
Because of the relative rotation between the vehicle chassis and the turntable, it has become customary in such vehicles to utilize hydrostatic steering for the vehicle. Hydrostatic steering consists of a hydrostatic steering valve that is located in the operator's compartment of the vehicle and is connected to a fluid ram which is located between the vehicle chassis and the steered front wheels for the vehicle.
Hydrostatic steering simplifies the interconnection between the steering wheel and the steered wheels at the front end of the vehicle because the connection can be made through conduits and a hydraulic swivel which is normally interposed between the turntable and the vehicle chassis. This permits 360 degree rotation of the turntable or upper crane unit with respect to the chassis frame.
Since hydrostatic steering is accomplished through the transfer of fluid from a pressurized fluid source through the hydrostatic steering valve to the fluid ram, loss of hydraulic power immediately reduces the control of the vehicle to a small fraction of that in existence when hydraulic power is available because all of the fluid transfer must be made by the forces applied by the operator to the steering wheel. Thus, when hydraulic power is lost during highway travel or near other traffic, an immediate dangerous condition is developed which can result in serious accidents. Thus, construction equipment manufacturers which utilize hydrostatic steering normally limit vehicle speeds to a maximum of 25 to 30 miles per hour.
Such limitation on the speed of the vehicle for transportation along major highways or other well travelled roads results in a substantial loss of operating time which is important to any construction equipment user, particularly in heavy duty equipment of this type.